In Traditional world, Testers are measured based on their contribution in finding critical defects. More the defects they find, higher the performance ratings are !

Will this work in Agile world? No way.

QA defects - invertionally proportional to the quality of features
Quality of features - proportional to quality of practices
Quality of practices - proportional to quality team
Quality of Team - proportional to the quality testers too

So, how come the testers job will be to find defects? Yes, the best agile development is "no defects after development"

Then, what is tester's job in agile?

Here are the responsibilties of a tester.
1. Find defects before even the first line of code is written
2. Find defects in the code written without even testing
3. Find defects in code immediately after the code is written

Are these possible? Definitely yes. How?

This paper is all about achieving these results. In some of the agile teams, we have achieved the results to a great extent.